Engine and vehicle fuel efficiency can be improved by lean burn internal combustion engines. To reduce emissions, these lean burn engines are coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, an additional three-way catalyst, known as a NOx trap or catalyst, is typically coupled downstream of the three-way catalytic converter, where the NOx trap is optimized to further reduce NOx. Both the three way catalyst and to a greater extent the NOx trap store NOx when the engine operates lean and release NOx to be reduced when the engine operates rich or near stoichiometry.
Accurate air-fuel ratio control allows optimized performance of this three way catalytic converter and NOx trap emission control system. One method to provide accurate air-fuel control uses a so-called universal exhaust gas oxygen sensor in the engine exhaust. This universal exhaust gas oxygen sensor provides an output that represents exhaust air-fuel ratio over a range, including lean, stoichiometric, and rich operation. Engine fuel injection is adjusted so that measured exhaust air-fuel ratio converges to a desired stoichiometric air-fuel ratio. Such a method is described in U.S. Pat. No. 5,473,887.
The inventors herein have recognized a disadvantage with the above approach. In particular, when such a system is applied to operation where exhaust air-fuel ratio is varied from lean to stoichiometric or rich, or exhaust air-fuel ratio is varied from stoichiometric or rich to lean, air-fuel ratio control is degraded. Consider the universal exhaust gas oxygen positioned downstream of the NOx trap. In this configuration, the sensor does not indicate the air-fuel ratio entering the NOx trap during some operating conditions. For example, when the air-fuel ratio entering the NOx trap is changed from stoichiometric or rich to lean, the sensor indicates stoichiometric until the oxygen storage capacity of the NOx trap is saturated. If this measurement is used for control, the engine will be operated excessively lean. In other words, in this example, the desired engine air-fuel ratio will be lean and the sensor will indicate stoichiometric. Since the sensor is used for feedback control, less fuel will be injected and the engine operated excessively lean until the oxygen storage capacity of the NOx trap is saturated.